Acoustic wave devices are used in mobile communication devices such as mobile phones. In recent years, the diversification and the high performance of the system of the mobile communication device requires the acoustic wave device to be reduced in size and height. Japanese Patent Application Publication Nos. 2002-100588 and 2012-186761 disclose a method of thinning a chip. The thinning of the chip allows the acoustic wave device to be reduced in size and height.
There has been a mounting method that makes the lower surface of the thinned chip face the upper surface of a substrate and flip-chip bonds the chip to the substrate by using a bump. When the aforementioned mounting method is applied to the acoustic wave device, an underfill cannot be formed between the upper surface of the substrate and the lower surface of the chip. This is because an excitation electrode exciting an acoustic wave is formed on the lower surface of the chip. Covered with the underfill, the excitation electrode is unable to excite the acoustic wave. The underfill is not formed in the acoustic wave device, and therefore the chip strength when the chip is pressed from above is low.
To enhance the strength of the chip, it is considered that the number of bumps used in flip-chip bonding is increased. However, as the number of bumps increases, the chip area increases. In addition, the stability of bump bonding decreases. Furthermore, characteristics are affected. Therefore, it is desired to enhance the chip strength without increasing the number of bumps.